The invention relates to a mirror for the EUV wavelength range and to a method for producing such a mirror. Furthermore, the invention relates to an EUV light source, an EUV illumination system and an EUV projection lens for microlithography comprising such a mirror. Moreover, the invention relates to a projection exposure apparatus for microlithography.
Projection exposure apparatuses for microlithography for the EUV wavelength range have to rely on the assumption that the mirrors used for the exposure or imaging of a mask into an image plane have a high reflectivity since, firstly, the product of the reflectivity values of the individual mirrors determines the total transmission of the projection exposure apparatus and since, secondly, the light power of EUV light sources is limited. In this case, the EUV wavelength range is understood to be the wavelength range of light having wavelengths of between 5 nm and 20 nm.
Mirrors for the EUV wavelength range around 13 nm having high reflectivity values are known from DE 101 55 711 A1, for example. The mirrors described therein consist of a layer arrangement which is applied on a substrate and which has a sequence of individual layers, wherein the layer arrangement comprises a plurality of layer subsystems each having a periodic sequence of at least two individual layers of different materials that form a period, wherein the number of periods and the thickness of the periods of the individual subsystems decrease from the substrate toward the surface.
What is disadvantageous about such mirrors, however, is that, over the entire lifetime of an EUV projection exposure apparatus, said mirrors absorb approximately ⅓ of all EUV photons incident on the mirror in the layer arrangement of the mirror. In general, the absorption of the high-energy EUV photons takes place by way of the photoelectric effect, electrons in the solid being ejected. As a consequence thereof, a large number of atoms having destabilized or broken chemical bonds are produced within the layer materials. Such atoms having destabilized bonds can then readily perform a change of site or location on an atomic scale, as a result of which the structure of the affected layer and thus also the optical property thereof changes. In initial experiments for continuous irradiation, a spectral shift of EUV mirrors was already ascertained.
The exact processes on account of the destabilized and broken bonds on an atomic scale are currently unclear. It is conceivable that the layer materials assume a state of increased density, which can explain the spectral shift ascertained. Such processes, described by the term “compaction”, are known for quartz glasses and for mirror layers in VUV microlithography with 193 nm. However, it is also conceivable that the destabilized atoms undergo a chemical reaction with atoms from adjacent layers or with atoms from the residual gas atmosphere of the projection exposure apparatus.
As a result of the structural change brought about by the destabilized atoms in the affected layer, the layer stress thereof and the surface roughness thereof also change besides the optical property.
In order to set the layer stress of a mirror, during the production thereof, so-called buffer layers or anti-stress layers (ASL) are usually applied between substrate and reflective coating, which compensate for the compressive stress of the reflective coating with their tensile stress. However, if the stress ratios within the layer arrangement change over the course of time as a result of the destabilized atoms in a mirror, then this inevitably leads to an impermissible change in the shape of the mirror surface. This then gives rise to impermissible image aberrations of the projection exposure apparatus.
In order to avoid stray light losses, mirrors for the EUV wavelength range are provided with very smooth substrate and layer surfaces during production. However, if the destabilized atoms give rise over the course of time to rough interfaces of the layers in the HSFR spatial frequency range, then this leads to stray light losses and thus to a loss of total transmission in a projection exposure apparatus, see U. Dinger et al. “Mirror substrates for EUV-lithography: progress in metrology and optical fabrication technology” in Proc. SPIE Vol. 4146, 2000, in particular for the definition of the surface roughness in the HSFR range with spatial wavelengths of the roughness of 10 nm to 1 μm and in the MSFR range with spatial wavelengths of the roughness of 1 μm to 1 mm.
Furthermore, the destabilized atoms at the interfaces of the layers can also enter into new chemical bonds, as a result of which the interdiffusion of the layers, which already takes place anyway, is intensified and/or as a result of which the effect of the layers used for suppressing the interdiffusion (so-called barrier layer) is reduced. An increased interdiffusion leads to a loss of contrast at the interfaces and this therefore leads to a loss of reflectivity in the mirror overall.